1. Technical Field
The present invention relates to rotors for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators. In addition, the invention can also be applied to industrial machines and household electrical appliances.
2. Description of Related Art
There are known electric rotating machines which include an Interior Permanent Magnet (IPM) rotor.
As shown in FIG. 7A, the IPM rotor 100 includes a rotor core 101 that has a plurality of slots 102 formed therein and a plurality of permanent magnets 103 each of which is inserted in a corresponding one of the slots 102 of the rotor core 101.
Each of the permanent magnets 103 has a substantially rectangular cross section perpendicular to the axial direction of the rotor core 101 (i.e., the direction perpendicular to the paper surface of FIG. 7A). Accordingly, each of the permanent magnets 103 has four corner portions including a first corner portion 103a. 
Among the four corner portions of the permanent magnet 103, the first corner portion 103a is positioned closest to an outer circumferential surface (or a radially outer surface) 100a of the rotor core 101; the outer circumferential surface 100a faces a stator (not shown) of the electric rotating machine which is disposed radially outside of the rotor 100.
Moreover, among the four corner portions of the permanent magnet 103, it is easiest for a demagnetizing magnetic field to concentrate on the first corner portion 103a. Here, the demagnetizing magnetic field denotes a magnetic field which is applied by the stator to the permanent magnet 103 in a direction opposite to the magnetization direction of the permanent magnet 103. Consequently, the first corner portion 103a is most likely to be permanently demagnetized by the demagnetizing magnetic field.
To prevent the demagnetizing magnetic field from concentrating on the first corner portion 103a, there is disclosed, for example in Japanese Patent Application Publications No. 2008-148391 and No. 2003-143788, a technique of providing an air hole around the first corner portion 103a. 
Specifically, according to the technique, as shown in FIG. 7A, a gap 105 is formed between a radially outer end portion (i.e., an end portion on the first corner portion 103a side) of a first side surface 103b of the permanent magnet 103 and the inner surface of the corresponding slot 102 (i.e., the inner surface of the rotor core 101 which defines the corresponding slot 102) in the magnetization direction of the permanent magnet 103. The first side surface 103b extends perpendicular to the magnetization of the permanent magnet 103 and makes up part of the first corner portion 103a of the permanent magnet 103.
With the gap 105, it becomes difficult for the demagnetizing magnetic field to concentrate on the first corner portion 103a. However, at the same time, the volume of the rotor core 101 is reduced by the gap 105. Consequently, it becomes easy for magnetic saturation to occur in the rotor core 101 in the vicinity of the gap 105.
Referring further to FIG. 7B, when an area A of the rotor core 101 in the vicinity of the first corner portion 103a of the permanent magnet 103 is magnetically saturated, it is impossible for magnetic flux from the stator to flow through the magnetically-saturated area A. Consequently, the magnetic flux from the stator comes to flow into the permanent magnet 103 via a dead end 105a of the gap 105. As a result, the demagnetizing magnetic field comes to concentrate on a given portion of the permanent magnet 103 in the vicinity of the dead end 105a of the gap 105.
That is, according to the above technique, concentration of the demagnetizing magnetic field is prevented from occurring at the first corner portion 103a of the permanent magnet 103, but instead occurs at the given portion of the permanent magnet 103 which is positioned close to the first corner portion 103a. As a result, the maximum strength of the demagnetizing magnetic field in the permanent magnet 103 is increased.
Generally, the thickness of the permanent magnet 103 in the magnetization direction is set based on the maximum strength of the demagnetizing magnetic field in the permanent magnet 103, so as to reliably prevent the permanent magnet 103 from being permanently demagnetized by the demagnetizing magnetic field.
Accordingly, with the increase in the maximum strength of the demagnetizing magnetic field in the permanent magnet 103, the thickness of the permanent magnet 103 in the magnetization direction is also increased. As a result, the volume and thus the weight of the permanent magnet 103 are accordingly increased, thereby increasing the cost of the rotor 100.